Automatic-frequency-correction device



Feb. 5, 1957 J. VAN BLADEL.

AUTOMATIC-FREQUENCY-CORRECTION DEVICE Filed March 1o, 1955 1E. w Y.

AUTOMATIC-FREQUENCY-CORRECTON DEVICE .lean Van Bladel, Brussels,Belgium, assigner to Hartford Nationtal Bank and Trust Company,Hartford, Conn., as trus ee Application March 10, 1953, Serial No.341,549 Claims priority, application Belgium March 1S, 1952 3 Claims.(Cl. Z50-36) The present invention relates to a circuit arrangement forautomatic frequency correction of an oscillator. More particularly, thisinvention relates to an arrangement for automatic frequency correctionof an oscillator, in which a control-Voltage obtained with the aid of adiscriminator circuit controls a frequency corrector coupled to theoscillator via a smoothing ilter comprising an output capacitor. Theinvention relates, more particularly, to anautomatic-frequency-correction (A. F. C.) circuit of the kind in whichbetween the output capacitor of the smoothing lter and the discriminatorcircuit provision is made of an interrupter, which is opened in order tomaintain the oscillator frequency of the moment, irrespective ofvariations in the Output voltage of the discriminator circuit.

l'n circuit arrangements of the aforesaid kind it is desired, inpractice, to cause the oscillator to oscillate for an appreciable time(for example, for a few minutes or a few tens of minutes), after theinterrupter has been opened, the frequency being the momentaryoscillator frequency at the instant of interruption. In this case, inaccordance with the use of the arrangement, the variation in frequency,or even in phase, relative to the oscillations of the oscillator at theinstant of interruption must be smaller than that occurring in the knownA. F. C. circuits of the present kind.

The invention has for its object to provide an A. F. C. device, whichmay largely fuliill the requirements stated in the preceding paragraphin a manner particularly simple in practice.

According to the invention, for this purpose the output terminals of thesmoothing filter are connected to the control-grid circuit of anelectrometer tube circuit.

The electrometer tube circuit preferably comprises an amplifying tubehaving a screen-grid, the operational voltages of Vwhich are such thatthe negative grid current is not more than u10 amperes, and the anodevoltage and the screen-grid lvoltage of ywhich are not more than volts.l

'ln'order thatthe invention may be readily carried into effect, it'willnowbefdescribed in detail with referencey to the accompanying drawing,`wherein:

Fig. l is a schematic diagram of an embodiment of the circuitarrangement of the present invention; and

Fig. 2 is a circuit diagram of an embodiment of the circuit arrangementof the present invention.

ln Fig. 1, according to the invention, reference numeral l designatesanoscillator, the frequency and phase of which are' stabilized relativelyto a control-oscillation applied to terminal 2. The control-oscillationand the oscillations produced by the oscillator are supplied, for thispurpose, to a discriminator circuit 3, constituted by a phase detector,which supplies a control-voltage which is fed to a smoothing filtercomprising two filter sections Ltand 4f. The A. F. C. control-voltageacross the output capacitor'of the filter section 4' controls frequencycontrolling or `determining means such as a reactance tube 6, via astage 5 (to be described more fully hereinafter),

which tube may, for example, be connected in parallel with thefrequency-determining circuit of the oscillator 1.

The arrangement 1, 2, 3, 4, 4', 6 is of a kind known per se and need notbe further explained.-

In the embodiment shown an interiupter 7 is provided between the filtersections 4 and 4'; this interrupter can be opened in order to maintain amomentary oscillator frequency, irrespective of variations in the outputvoltage of the discriminator circuit 3. In the open position of theinterrupter 7, shown in Fig. 1 by a broken line, the charge of theoutput capacitor of the filter section 4' constitutes an A. F. C.control-voltage for the reactance tube 6, independent of the outputvoltage of the discriminator 3. The charge of the capacitor, and hencealso the corrected oscillator frequency, in known circuit arrangementsvaries due to leakage currents; the velocity of variation in thecapacitor charge varies with the time constant of the capacitor circuit,which may, for example, be 10 to 2O seconds, if the operation is carriedout carefully.

According to the invention the variation in the charge oi the capacitormay be substantially avoided by connecting the output terminals of thefilter section 4 to the control-grid circuit of an electrometer tubecircuit 5, which constitutes a pre-amplifying stage preceding thereactance tube in the embodiment shown.

ln this circuit-arrangement the variation in the charge of the capacitordepends primarily on two factors, i. e. the grid current passing throughthe control-'grid circuit of the electrometer tube circuit and the leakresistor of the capacitor used. The effect of the last-mentioned factorneed not be explained more fully; it may be reduced to a very smallvalue by using capacitors of good quality, for example, of the typeDubilier Nitrogol.

When an electrometer tube circuit is utilized the occurrence of gridcurrents, due to ionization, deposit of the residual gases in the tube,secondary-electron emission at the tube electrodes, photo-emission, ionemission at the cathode and the like in the case of a normal tubearrangement, is greatly reduced to values of less than 10-10 amperes(cf. the book National Nuclear Energy Series, Manhattan Projects,Technical Section, Division V, vol. l, Electronics ExperimentalTechniques, by W. C. Elmore and M. Sands, pages 180 to 187; McGraw-HillPublishing Company, first edition). The charge of the capacitor-issubstantially not affected by this excessively low grid current or, inother terms, a very high effective time constant may be obtained for thecapacit-or circuit, if the intermpter or switch 7 is opened. Thus avariation in the oscillator frequency, due to a variation in the chargeof the capacitor, is substantially avoided. Even in the embodimentshown, in which a phase detector is used as a discriminator, it ispossible for the phase relationship between the oscillator oscillationsand the con' trol-oscillation to be maintained for an appreciable timafterl the switch 7 hasbeen opened.

For the electrometer tube circuit use may be made of specialelectrometer tubes, but, as an alternative, for this purpose certaintypes of the standard radio-receiving tubes may be used; for example,Philips type DL 72 or DL 92. These radio-receiving tubes are used withgreatly reduced supply voltages, i. e. with anode voltages andscreengrid voltages of not more than 15 volts. The filament current ofthe directly heated cathode for direct current must be considerablyreduced, for example, to 5() to 70% of the normal operational value.

With the special electrometer tubes, for example, Philips type 4060,4G65, 4G65, slightly'higher supply voltages for the anode' and thescreen-gridare sometimes permissible, for exam-ple voltages of '25 to 30volts.

With the use of the tubes referred to negative grid currents of not morethan *-12 to l0-13 amperes may be readily obtained in practice. Itshould be noted' that without the use of a capacitor of excellentquality and with a simpleconstruction of the. electrometer tubearrangement, effective time constants. of afew hours. may Ibe obtained'in practice.

Fig. 2 is a circuitdiagramof an embodiment of the invention. Thisembodiment is assumed to be used more particularly for studyingvthephase variation, in this case, phase uctuations in the alternatingsupply line voltage in a high-currentpower station after the occurrenceof a disturbance, due for example, to short-circuit. In Fig. 2relatively crosswise coupled-pentodes S and 9, 4which vare connected'viaan lanode resistor lll and a series combination of resistors island 12respectively to a positive terminal l`3` of an anode voltage battery,constitute Ian RC- oscillator of a' kind known per se. 'Thecathode'ofthc pentode 9' is connected to ground.' via `a cathode'resistor 14 and the screen-grid voltages of the tubes il `and 9arederived. from screen-grid resistors 1.5 and 16. The anode of thepentode-S is connected viaa blocking capacitor 17 and a. gridf leakresistor 15 to the control-grid of the pentode 9. A junction' 19 of theanode resistors 11 and l2 is connected via a blocking capacitor 20 andthe series combination of a resistor 21 and a capacitor 22 to thecontrol-grid of the pentode 8, which is connected to ground through aseries combination of a capacitor 24 'and' a variable resistor 25;,shunted by a resistor 23. ln order to improve the frequency stability ofthe RC- oscillator 8, 9 described so far, a negative feed back voltageis supplied tothe cathode of the tube 8; The feedback voltage is derivedfrom a potentiometer, connected in` parallel with the branch Zit-25;this potentiometer comprising the series combination of a variableresistor 26, a resistor 27' und a voltage-dependent resistor 23,constituted by a filament current lamp.

The frequency of the sine oscillations produced by the oscillator 8, 9,which is determined primarily, as is known, by the elements of thebranch 2125, is `adjusted to a value which is lower than the normalsupply line voltage frequency cycles per second.

In the embodiment shown, the frequency andthe phase of` the oscillatorvoltage are stabilized on the supply line voltage. For this purpose theoscillator voltage derived from the anode resistors l1, 12 of theRC-o'scillator is supplied, vsubsequentto amplification andv phasereversal in an amplifying stage 2?, together with the supply -linevolt-agey from a terminal 30, lto atpha'se detector constituted by anormally cut-off hexode mixing tube 31. in the phase detector shown thehexode tube 31.is released only if the oscillations originating from theoscillator 8, 9 and the supply line voltages have simultaneouslypositive polarity, and the amplitudes of these two voltages are chosento be so `high that amplitude limitation occurs. Thus a control-voltageis produced across an output resistor 32 of thc tube 3l. Thiscontrol'voltage has the shape of `trapezoidal pulses of negativepolarity; the duration of the large trapezium bases beingsubstantiallyproportional to the absolute value of the phase difference between theoscillator voltage und the supply line voltage.

In the embodiment shown, the pulsatory discriminator voltage is suppliedvia a potentiometer 33, 34, connected to the anode resistor 32, andthrough a rectifying circuit 35, 36, to a low-pass filter 37,suppressing the pulse recurrence frequency and comprising the seriesAcombination of 'three sections. The first and second filter sectionseach comprise a series resistor 3S and 39 respectively and a parallelimpedance constituted by the series combination of a resistor 45t land4l respectively and alcapacitor 42 und 43 respectively. The-'thirdfilter section is constituted 'by a series resistor 44 and a parallelcapacitor'tlS. The parallel resistors itl and 4l, serve to improve thecontrol stability, more particularly, toV restrict an over-shoot. Acrossthe output capacitor 450i the lowpassl filter 37 occurs a smoothed A. F.C. control-voltage, which is supplied via a series resistor 46 tolthecontrol-grid of a pentode 47, connected as a frequency corrector, inan electrometer tube arrangement. The anode-cathode discharge path ofthe pentode 47 constitutes a variable resistor varying With thecontrol-voltage supplied to the control-grid and connected through aresistor 43 and a blocking capacitor 49, in` parallel with the network23 to 25 of the frequency-determining circuit 21 to 25; of theoscillator 8, 9.

The frequency-corrector arrangement is controlled by the A. l?. C.control-voltage ina manner such that lsynchronism occurs between thesupply line voltage-and the oscillations produced by the oscillator 8,9. However, a phase displacement, the value of which varies with thevalue of the required frequency correction, remains between `the twosynchronized oscillations.

In order to maintain a momentary oscillator frequency, irrespective ofvariations in the output of the discriminator circuit, anvinterrupter 50is provided between the junction of the series resistor 44 yand theoutput capacitor 45 of the smoothing filter 37. The interruptor 50 formspart of a relay 51.

In order to avoid a variation in the voltage of the output capacitor 45`and hence in the frequency, and, in the present case, in the phase ofthe oscillator 3, 9, when the interrupter Si) is opened, the pentode 47,operating as a frequency corrector, is used in the electrometer tubearrangement. The supply-voltages of the pentode 47 are derived from apotentiometer comprising the series combination or twoparallel-connected resistors 52v and 53, a' resistor 54y and a resistor'55. This series combination is connected on the one hand to the positiveterminal 13 of the' 'anode voltage supply and on the other hand toground via the filament current circuit' of the pentode 47. The.junction of the parallel-connected' resistors 52 yand 53 and theresistor Sti is' connected to the screen-grid of the pentode' 47 andthejun'ctiono'f the resistors 54 and 55 is connected through a resistor 56to the anode of the' pento'de 47. in this circuit-arrangement theresistors of `the potentiometer are so' proportioned that the anodevoltage and the screen-grid voltage are 9 and 14 volts respectively,while the filament current of the directly heated cathode, which isnormallyfor example, SO'milliampere's, is lreduced to 30 mA.

At the instant when the interruptor 50 is' opened, owing tov adisturbance inthe supply lines, a certain phase displacement is producedbetween the oscillator voltage and the supply line voltage. For acertain period of time thereafter, it is desired that the oscillator goon oscillating, as if the connection with the undisturbed supply lineswere not interrupted, in order to permit the utilization of theoscillator oscillations to make known the phase variation of the supplyline voltage in case- 0f the disturbance. The phase variation can bereadily dctected by connecting the anode circuit of the phase detector31 via a blocking capacitor 57 and a series resistor 58 to thecontrol-gridv of an amplifying tube S9, connected as a limiting device,and then by connectingradifferential indicating instrument 60l totheoutput circuit of amplifying tube 59.. The` differentialV instrument 60is controlled by a comparison voltage from a potentiometer 61, 62,connected to the positive terminal 0f the anodel supply source. In orderto study the phase variation, in this case, phase fluctuations of thesupply line voltage after' a disturbance has occurred in thesupply'lines, the interruptor Si) is opened. After this the duration ofthe output pulses of thephase detector 3l constitute a measure for thephase variations, which may be followed and read on the differentialinstrument 60. The phase variation to be measured in practice may be amultiple of l cycle per second, hencea multiple of 360 degrees, duringthe first live seconds.

In the arrangement of. Fig. 2,' in operation, a' phase variation. ofless than 10 degrees occurred owing to variations in the charge of thecapacitor during the subsequent tive seconds after the interrupter hadbeen opened.

Operable component values for the circuit arrangement of Fig. 2 may beas follows.

The anode voltage source may furnish 250 volts at 55 milliamperes.

Oscillator 8, 9

Tube 8 EF 42 Tube 9 EF 42 R10 5.6 kilohms R11 5.6 kilohms R12 5.6kilohms R14 560 ohms R15 27 ohms R16 27 Ohms R18 1 megohm R21 33 kilohmsR23 54 kilohms R25 2.5 kilohms R26 10 kilohms R21 2.7 kilohms R28 lamp220 volts, 10 watts C17 0.1 microfarad C20 8 microfarads (electrolytic)C22 0.1 microfarad C24 0.1microfarad Electrometer tube circuit Tube 47DL 92 R46 1 megohm R48 120 kilohms Rs2 15 kilohms (wire resistance) R5315 kilohms (wire resistance) R54 `150 ohms (wire resistance) R55 220ohms (wire resistance) R55 39 kilohms Band pass lter Rza 470 kilohms R39330 kilohms R40 100 kilohms R41 82 kilohms R44 180 kilohms C42 0.1microfarad C43 0.25 microfarad C45 1 microfarad It is to be understoodthat the invention is not limited to the details disclosed but includesall such variations and modications as fall within the spirit of theinvention and the scope of the appended claims.

What is claimed is:

1. A circuit arrangement for automatic correction of the output voltage.of an oscillator comprising a discriminator circuit adapted to providea control voltage for controlling the output voltage of said oscillator,means for supplying said oscillator output voltage to said discriminatorcircuit, means for supplying a control oscillation to said discriminatorcircuit, a smoothing tilter having an output capacitor, an interrupterinterposed between said discriminator circuit and said output capactoradapted to be opened at given intervals of time in order to maintain amomentary oscillatory frequency at the output of said oscillatorirrespective of the variations of the control voltage provided by saiddiscriminator circuit, means coupled to said oscillator for controllingthe frequency thereof, an electron discharge device having an anode, acathode and a control grid interposed between said smoothing lter andsaid frequency controlling means, said electron discharge device havinga maximum negative grid current of 10-10 amperes, said output capacitorbeing connected to said control grid, and an anode-cathode dischargepath for said discharge device comprising a variable resistor varying inaccordance with the magnitude of said control voltage, said variableresistor being connected in parallel with said frequency controllingmeans.

2. A circuit arrangement for automatic correction of the output voltageof an oscillator comprisng a discriminator circuit adapted to provide acontrol voltage for controlling the output voltage of said oscillator,means for supplying said oscillator output voltage to said discriminatorcircuit, means for supplying a control oscillation for saiddiscriminator circuit, a smoothing filter having an output capacitor, aninterrupter interposed between said discriminator circuit and saidoutput capacitor adapted to be opened at given intervals of time inorder to maintain a momentary oscillatory frequency at the output ofsaid oscillator irrespective of the variations of the control voltageprovided by said discriminator circuit, means coupled to said oscillatorfor controlling the frequency thereof, an electron discharge devicehaving an anode, a cathode, a control grid and a screen grid interposedbetween said smoothing lter and said frequency controlling means, saidelectron discharge device having a maximum negative grid current ofl0"lo amperes, said output capacitor being connected to said controlgrid, a source of anode supply voltage, potentiometer means connectedbetween said source, said screen grid, said anode and said cathode formaintaining a maximum anode voltage and a maximum screen grid voltage of15 volts, and an anode-cathode discharge path for said discharge devicecomprising a variable resistor varying in accordance with the magnitudeof said control voltage, said variable resistor being connected inparallel with said frequency controlling means.

3. A circuit arrangement as claimed in claim 2, wherein saiddiscriminator circuit comprises phase detecting means controlled by saidoscillator output voltage and said control oscillation.

References Cited in the le of this patent UNITED STATES PATENTS2,437,609 Mayle Mar. 9, 1948 2,479,817 Curran Aug. 23, 1949 2,510,095Frankel June 6, 1950 2,567,286 Hugenholtz Sept. 11, 1951 2,715,660Colchester Aug. 16, 1955

